Minimizing Retention Induced Refresh Through Exploiting Process Variation of Flash Memory

Refresh schemes have been the default approach in NAND flash memory to avoid data losses. The critical issue of the refresh schemes is that they introduce additional costs on lifetime and performance. Recent work proposed to minimize the refresh costs by using uniform refresh frequencies based on the number of program/erase (P/E) cycles. However, from our investigation, we find that the refresh costs still have a high burden on the lifetime performance. In this paper, a novel refresh minimization scheme is proposed by exploiting the process variation (PV) of flash memory. State-of-the-art flash memory always has significant PV, which introduces large variations on the retention time of flash blocks. In order to reduce the refresh costs, we first propose a new refresh frequency determination scheme by detecting the supported retention time of flash blocks. If the detected retention time is large, a low refresh frequency can be applied to minimize the refresh costs. Second, considering that the retention time requirements of data are varied with each others, we further propose a data hotness and refresh frequency matching scheme. The matching scheme is designed to allocate data to blocks with right higher supported retention time. Through simulation studies, the lifetime and performance are significantly improved compared with state-of-the-art refresh schemes.